In conventional file systems, a client on a workstation can access a specific named volume on a specific named server. As the volume name is human-readable, there could be other volumes in the network with the same name on different servers, but these bear no relation to any other volumes with the same name. Products exist to replicate data between volumes on different servers, but the client is still required to specify which instance of a replicated volume it wants to access.
A Distributed File System (DFS) eliminates the strong tie between a file and the server on which it resides. With DFS, volumes still exist, but they can move between servers or have multiple instances that exist on different servers. The client specifies only the DFS volume name or its Global Unique Identifier (GUID) when accessing files. The advantages of DFS can be generalized to any kind of resource that can be distributed (e.g., printers, scanners, etc.). But without some mechanism to assist the client in finding to which physical server or servers the resource is attached, DFS is of limited value.
There have been several prior attempts to implement distributed file systems. The National Software Works (NSW), implemented by the Advanced Research Projects Agency (ARPA), included a single global volume distributed across multiple physical machines for file storage, a solution that did not scale well. Cronus, a distributed operating system research project undertaken by Bolt Beranek and Newman (BBN) under contract to the Rome Air Development Center (RADC), used a (statistically) unique name for the object as a clue to its location. But if the object was not known by that host (perhaps because it had been moved), the object would have to be located by broadcasting a message on the network. This approach did not scale well, and broadcasting messages can be difficult in any event. The AFS-3 file system by Transarc Corporation included a single back-end database implementation with a well-known name for the volume location servers. This approach is difficult to generalize, and has a single point of failure (the database). Microsoft used reparse points, which contain the full list of hosts where the volume instances can be found. But if a volume moves or a new instance is added, the reparse points must be modified, which is a difficult task.
Accordingly, a need remains for a mechanism that allows a client to locate instances of a resource given the resource's naming information that is easily scalable, includes redundancies for continued performance, and is easily updated as hosts and resource instances are added to or removed from the DFS.